1. Field of the Invention
The present invention relates to a pattern search method suitable for being used for measurement of a circuit pattern formed on a semiconductor wafer. More particularly, it relates to a pattern search method for detecting the circuit pattern by a pattern matching scheme.
2. Description of the Related Art
At semiconductor fabrication processing steps, line width or the like of a circuit pattern formed on a semiconductor wafer is measured by a measurement device including a scanning electron microscope. In the measurement of the circuit pattern like this, it is required to detect a measurement position on the image of the circuit pattern.
From conventionally, a pattern matching scheme based on a template has been known as a method for detecting a predetermined measurement position from the image of a circuit pattern. According to this scheme, a small image region including a measurement target is cut out from the image of the circuit pattern, then registering the small image region as a template in advance. When an image including the measurement target is given at the measurement processing step, the scanning is performed while comparing the template with the image. Moreover, degree of similarity between the template and a screen region with which the template is compared will be checked. Then, if the degree of similarity is found to be high, it is judged that the screen region is the image region including the measurement target.
In recent years, the following technologies have been introduced: Short-wavelength implementation for light-exposure laser of light-exposure machine, and novel technologies on light-exposure mask (such as phase shift mask and OPC). If a circuit pattern formed under the formation conditions like this is observed with a high magnification, there appears a gently curved and low space-frequency configuration. Namely, characteristics of the pattern are dispersed. In particular, in the case of a memory cell or the like, there often appears the gently curved pattern or a repeated pattern.
In conventional methods, if the pattern of the image region cut out as the template exhibits only a small difference when compared with pattern of another image region, the position detection becomes difficult, or an error occurs.
In an embodiment disclosed in JP-A-6-120310, a small region including characteristic portion of a measurement target is cut out, then registering the small region as a first template. Also, a region including the entire measurement target is cut out, then registering the region as a second template. When an observation image is given, at first, the measurement target is detected by a pattern matching based on the first template. If, however, the region cut out as the first template extends off the observation image, a pattern matching based on the second template is performed, thereby detecting position of the measurement target. Then, if the position of the measurement target has been detected, the accurate position is identified based on the first template.